Display Signal Conversion Apparatus

ABSTRACT

A display device includes a display panel including red, blue, green and white (RBGW) sub pixels, a signal conversion apparatus for converting an input signal having a first color format and color arrangement into a display signal having a second color format and color arrangement, a color conversion unit configured to convert the first color format into the second color format; a sub pixel position determining signal which determines a color arrangement of a sub pixel position of the display panel, and a sub pixel signal generation unit configured to generate a sub pixel signal having the second color format and color arrangement based on the sub pixel position determining signal.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. application Ser. No. 12/756,534, filed Apr. 8, 2010, the contents of which are incorporated hereinby reference.

The present application claims priority from Japanese Application JP2009-095951 filed on Apr. 10, 2009, the content to which is herebyincorporated by reference into this application.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a display signal conversion apparatusparticularly to a signal conversion from an RGB input signal to an RGBWdisplay signal.

2. Description of the Related Art

As described in “New-Edition Color Science Handbook, 2nd edition,”edited by the Color Science Association of Japan, University of TokyoPress, originally printed in 1998, a color image may be constituted byRGB (red, green, and blue) three color signals. Further, a smallest unitof a color image is called a pixel. A screen that is a set of pixels iscalled image data, a video signal, a color signal, or the like dependingon fields. In the following description, those terms may be used in amixed manner.

Display devices based on various principles have been developed for thepurpose of displaying a color image by supplying a video signal. Amongthem, a liquid crystal panel has a large number of liquid crystalelements for controlling transmittance, which are arranged in a plane soas to constitute a screen. In this display principle, RGB three colorsmay not be disposed at the same position, and hence color filters of RGBthree colors are shifted regularly. A minimum unit of each color displayis called a sub pixel, and a set of adjacent RGB sub pixels is called apixel.

In the following description, the terms of pixel and sub pixel may beused in a mixed manner. As described above, for a mechanical reason ofthe display device, a type of color and a position of color should beconsidered. Practically, misregistration of sub pixels may not berecognized in many cases, but may be perceived as a smoothness of acontour of a letter or a figure in some cases of a mobile terminal orthe like with a short visual distance.

A video signal distributed in common is expressed by RGB three colorsper pixel or by a signal that may be converted into RGB three colors.For instance, a video signal of a color television is constituted by aluminance signal and two types of color-difference signals, which may beconverted into RGB three colors by simple conversion equations.

A video signal distributed in common may have the number of pixels thatis different from the number of pixels included in the screen dependingon a broadcasting type. For instance, the number of pixels of a colortelevision video signal is different between analog terrestrialbroadcasting and satellite high definition broadcasting. Therefore, thenumber of pixels is converted so that display may be performed on thesame display device. In this way, in order to adjust a differencebetween the input video signal and characteristic of the display device,it is known to perform signal processing such as conversion of a colortype, conversion of a color arrangement (sub pixel rendering), and thelike.

The liquid crystal panel that performs transmittance control is combinedwith a light source so as to function as a display. A type of the lightsource for use is not limited, but the following description exemplifiesthe case where a light emission unit (backlight) is disposed on theopposite side to the display screen.

A color image to be displayed is generated by combining a wavelengthcharacteristic of the backlight and wavelength characteristics of subpixels of the RGB three colors. As combined transmittance thereof ishigher, light emission of the backlight may be used more efficiently.

JP 60-61724 A discloses a technology for improving the transmittance byproviding four color sub pixels including white (W) color in addition tothe above-mentioned RGB three colors. Because a color filter is notnecessary for W sub pixels, the transmittance is improved three times ascompared with the transmittance of RGB sub pixels. The RGB sub pixelsand the W sub pixel are arranged in an appropriate order so that thetransmittance may be improved.

In addition, JP 2004-538523 A discloses an example of signal processingin which a sub pixel arrangement conversion (sub pixel renderingprocess) is noted. JP 2004-538523 A discloses a method of performing thesignal conversion by filtering process in which the signal conversionmethod is analyzed from a viewpoint of frequency components in the casewhere a pixel position of an input signal is different from a sub pixelposition of the display panel. The filtering process is a well-knownsignal processing technology defined by a convolution integral of theinput signal and a coefficient, which is characterized in coefficientsetting for the purpose of maintaining image quality accompanying a subpixel arrangement. In addition, when the filtering process is performed,gamma conversion for securing linearity of the signal is essential.

In addition, JP 2004-538523 A discloses, a correction technology for aresult of the filtering process operation using a monochrome fine linepattern detection unit on the assumption that sufficient image qualitymay not be obtained only by the above-mentioned filtering process(blurred fine line or color balance shift).

SUMMARY OF THE INVENTION

One object of one or more embodiments of the present invention isimproving brightness, i.e., realizing a method and an apparatus forconverting a first signal to be input to a second signal to be used fordisplay so as to reduce power consumption of a display panel byenhancing its transmittance.

The contents thereof involve color conversion from a color type of afirst signal to a color type of a second signal, and arrangementconversion from a color arrangement of the first signal to a colorarrangement of the second signal. In the following description, thefirst signal is a video signal of a television signal, or the like, andthe second signal is a signal for displaying on the display panel.

For example, a color type of the first signal is the RGB three colors,and a color type of the second signal is the RGBW four colors. Inaddition, the color arrangement of the first signal is set so as toassign RGB per pixel, and the color arrangement of the second signal isset so as to assign RG or BW per pixel. The minimum unit arranged in apixel is called the sub pixel, and the arrangement conversion by subpixel unit is called sub pixel rendering.

In the liquid crystal panel including RGBW sub pixels for the purpose ofimproving transmittance, a combination of RG sub pixels or BW sub pixelsis arranged as a pixel at a pixel position of the input signal. In thiscase, the display pixel including the RG sub pixels cannot display a Bsignal, and the display pixel including the BW sub pixels cannot displayRG signals.

For instance, if a fine line having a line width of one pixel isdisplayed by a single color (one of RGB colors), the fine line cannot bedisplayed every one pixel, which causes deterioration of image quality.If signal processing, which suppresses the degree of image qualitydeterioration based on whether input signal can be displayed or notdepending on the pixel arrangement of the display panel by means ofconverting the input signal to the display signal, is realized, it ispossible to obtain sufficient effect of improving the transmittance.From the above-mentioned viewpoint, when using the RGBW panel, there areproblems regarding the color signal conversion method from the RGBsignals into RGBW signal containing the W signal and the pixelarrangement conversion method depending on the pixel arrangement of thedisplay panel.

JP 60-61724 A discloses a structure of the liquid crystal panel in whichRGBW four-color sub pixels are arranged in a plane. The W sub pixel doesnot have a wavelength discrimination characteristic and transmits lightemitted from the backlight, and hence transmittance improvement effectmay be expected. However, JP 60-61724 A discloses no conversion methodfrom the RGB input signal into the RGBW output signal.

JP 2004-538523 A discloses an apparatus structure utilizing a so-calledfiltering process on the principle of signal processing based onfrequency components as the pixel arrangement conversion method from theinput signal to the display signal. In addition, JP 2004-538523 Adiscloses an apparatus structure utilizing gamma conversion so that thefiltering process may be valid as a linear operation.

Here, the filtering process is a well-known technology defined by aconvolution integral, and it is known as a natural technology to applythe filtering process to resampling of pixels. The gamma conversion is awell-known technology that is used for maintaining linearity of signal,and it is known as a typical technology to adjust a coefficient or aparameter thereof.

In order to calculate a signal value of a target pixel using thefiltering process, a sum of products between decimal fractioncoefficients and signal values of the target pixel and adjacent pixelsis calculated. In this case, if a calculation error remains, the erroris added to an image region, which causes image quality deterioration.

In order to prevent this deterioration, it is necessary that amultiplying circuit for multiplying together the decimal fractioncoefficient and the input signal, an adding circuit for adding productsof the multiplying operation, signal wires for transmitting signalsunder calculation, and the like have a sufficient number of bits.

As a simple example, in the filtering process for 3 by 3 pixels, a sumof (9) a result (12 bits each) of multiplying the input signal (8 bits)and the decimal fraction coefficient (4 bits) has 16 bits at most. Thiscorresponds to two times the number of bits of the input signal.

Further, where sufficient image quality may not be obtained only by theabove-mentioned filtering process (blurred fine line or color balanceshift), JP 2004-538523 A discloses a technology for correcting a resultof filtering process operation using a monochrome fine line patterndetection unit.

However, the pixel arrangement of the display panel is not considered inthe correction process. As described above, the filtering process basedon frequency components is effective while the filtering process is notso sufficient that the correction process is necessary. This lacksconsistency in technology, and causes complication and upsizing of thecircuit.

In the end, in order to realize the signal conversion circuit disclosedin JP 2004-538523 A in a practical manner, it is essential to handle thenumber of bits that is approximately two times larger than the inputsignal or the output signal by an internal circuit, which causescomplication and upsizing of the circuit. In addition, JP 2004-538523 Ais not directed to the display panel including W sub pixels, and doesnot disclose a color signal conversion method from the RGB input signalinto the RGBW output signal. Further, it is not certain whether or notthe generation method of the sub pixel signal disclosed in JP2004-538523 A may be applied to signal conversion of the RGBW panel.

In addition, JP 2004-538523 A discloses the signal processing based onfrequency components, which is subject to a constraint of the well-knownsampling theorem. In other words, it is theoretically impossible toreproduce correctly a sub pixel signal having a shorter period (higherfrequency) than a pixel period (frequency) of the input signal.

One aspect of the present invention has been made for solving theabove-mentioned problems of the conventional technologies, and it is oneobject of the present invention to provide a technology for converting afirst signal to a second signal by using a simple signal processingprocedure so as to improve transmittance while maintaining image qualityof a display panel having a second color arrangement.

The above-mentioned and other objects and novel features of the presentinvention become apparent from the following description and theaccompanying drawings.

A typical aspect of the invention disclosed herein is briefly outlinedas follows.

In order to solve the above-mentioned problems, according to one aspectof the present invention, a display signal conversion apparatus convertsa first signal having a first color type and color arrangement into asecond signal having a second color type and color arrangement (subpixel arrangement). The display signal conversion apparatus includes acolor conversion unit configured to convert the first color type intothe second color type, a pattern determination unit configured todetermine a pattern of one of the first color arrangement and the secondcolor arrangement, a unit configured to determine a color arrangement(sub pixel arrangement) of a pixel position of the second signalcorresponding to a pixel position of the first signal, and a sub pixelsignal generation unit configured to generate a signal (sub pixelsignal) having a second color combination and color arrangement based ona result of the signal pattern determination and the sub pixelarrangement.

In addition, in one aspect of the present invention, a phase between apixel position of the input RGB signal and a sub pixel arrangementposition of a display panel is constant. In other words, between inputand output of the color signal, pixel arrangement intervals (i.e., thenumber of pixels per screen) have a relationship of an integralmultiple. This corresponds, for example, to the case where any one of RGand BW sub pixels of the display panel are positioned at the pixelpositions of the input signal. On the contrary, although it is not atarget this time, the case where the phase is not constant correspondsto the case where the pixel arrangement intervals of the display panel(i.e., the number of pixels per screen) do not have a relationship of anintegral multiple with respect to the input signal.

The conversion from the input RGB signal to the RGBW signal for displayis split into two steps involving (1) color signal conversion(conversion from the RGB signal to the RGBW signal) and (2) signalconversion based on the pixel arrangement of sub pixels of the RGBWpanel of the display panel (sub pixel rendering), and the two steps areperformed in the order of (1) and (2).

One aspect of the present invention first performs (1) color signalconversion. As described above, it is a precondition that there is nophase shift between the input and the output of the color signal.Therefore, it is not necessary to consider the arrangement position ofsub pixels at this stage. In general, the color conversion is performedby multiplying the color signal of a pixel unit that is supplied in thescanning order by a decimal fraction coefficient.

Next, in the stage (2), the RGBW signal after the color conversionperformed in the above-mentioned step (1) is supplied, and the signalconversion is performed based on the arrangement position of sub pixelsof the display panel. At this stage, a unit configured to determine asignal pattern of a two-dimensional arrangement of the RGBW signal inthe target pixel and adjacent pixels is prepared so as to detect a fineline having high chroma saturation in particular. In addition, becausethere is no phase shift as described above, the positional relationshipbetween the input signal and the sub pixels is constant. Utilizing thisrelationship, it is possible to determine whether or not the color typemay be displayed. Further, a correspondence relationship between theabove-mentioned condition and the color signal of the sub pixelpositioned at the target pixel is prepared in advance for signalprocessing of the pixel arrangement conversion.

Further, the color signal of the sub pixel corresponding to the targetpixel is generated from the RGBW signal obtained by color conversion ofthe input signal, the signal pattern detected from the target pixel andadjacent pixels, and the sub pixel arrangement of the RGBW panelcorresponding to the pixel position of the target pixel.

Here, in the method of generating the color signal, signal processingcontents based on the combination of the above-mentioned conditions areprepared in advance so that the color signal that may not be displayeddepending on the sub pixel arrangement is distributed to a differentpixel (sub pixel) or different color to be displayed.

As described above, the method disclosed in JP 2004-538523 A uses thesum of products (integral) operation of the reference range so as tocalculate the sub pixel signal of the target pixel position. Incontrast, the procedure of one aspect of the present invention involvesdistributing the signal to a different pixel or a different color basedon the signal pattern. The method disclosed in JP 2004-538523 A notesfrequency components for the filtering process, and hence is subject toconstraints of the sampling theorem. In contrast, according to oneaspect of the present invention, the signal of the sub pixel unit may begenerated without being subject to constraints of the sampling theorem.

According to one aspect of the present invention, the pixel arrangementis converted by the single procedure, and the additional correctionprocess is not necessary although it is essential in JP 2004-538523 A.Because the multiplier (which is essential in JP 2004-538523 A) is notused as the operating unit of the above-mentioned step (2) in one aspectof the present invention, the number of bits handled by the operatingcircuit is approximately the same as the input signal or the outputsignal. In this way, the signal conversion is performed from the RGBsignal to the RGBW signal by using a simple signal processing procedure,so as to improve transmittance while maintaining image quality of theRGBW panel.

Effects obtained by the typical aspect of the invention disclosed hereinare briefly described as follows.

According to one aspect of the present invention, the signal conversionis performed from a first signal (for example, RGB signal) to a secondsignal (for example, RGBW signal) by using a simple signal processingprocedure, so as to improve transmittance while maintaining imagequality of a display panel (for example, RGBW panel) having a secondcolor arrangement.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIGS. 1A and 1B are diagrams illustrating a fundamental structure of adisplay signal conversion apparatus according to an embodiment of thepresent invention;

FIGS. 2A and 2B are diagrams illustrating a color type and a colorarrangement that are used in the display signal conversion apparatusaccording to the embodiment of the present invention;

FIG. 3 is a diagram illustrating a structural example of a pixelarrangement determination unit 151 illustrated in FIGS. 1A and 1B;

FIG. 4 is a diagram illustrating an example of how to set a thresholdvalue in the three-dimensional color space in the case of the RGB threecolors;

FIG. 5 is a diagram illustrating an operation of the signal patterndetermination unit 153 illustrated in FIGS. 1A and 1B;

FIG. 6 is a diagram illustrating an operation of the signal patterndetermination unit 153 illustrated in FIGS. 1A and 1B;

FIG. 7 is a diagram illustrating a circuit structural example of thepixel arrangement determination unit 151, a line memory 152, a signalpattern determination unit 153, and a signal distribution unit 154illustrated in FIGS. 1A and 1B;

FIGS. 8A and 8B are diagrams illustrating a pixel (sub pixel)arrangement of the display panel;

FIG. 9 is a diagram illustrating an operation of the signal distributionin the display signal conversion apparatus according to the embodimentof the present invention;

FIG. 10 is a diagram illustrating an example of a general structure of aliquid crystal display to which the display signal conversion apparatusaccording to the embodiment of the present invention is added as abridge circuit;

FIGS. 11A and 11B are diagrams illustrating a structural example of atelevision receiver using the display signal conversion apparatusaccording to the embodiment of the present invention;

FIG. 12 is a diagram illustrating a structural example of an apparatusfor performing luminance modulation of a backlight by using the displaysignal conversion apparatus according to the embodiment of the presentinvention;

FIGS. 13A and 13B are diagrams illustrating a sub pixel arrangement ofthe display panel; and

FIGS. 14A and 14B are diagrams illustrating a variation example of thefundamental structure of the display signal conversion apparatusaccording to the embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, embodiments of the present invention are described indetail with reference to the accompanying drawings.

Note that in all the drawings for illustrating embodiments, the elementhaving the same function is denoted by the same reference numeral orsymbol so that overlapping description is omitted.

FIGS. 1A and 1B illustrate a fundamental structure of a display signalconversion apparatus according to the embodiment of the presentinvention. FIGS. 2A and 2B illustrate color types and color arrangementsof signals that are described in the following description.

An input signal 110 that is a first signal includes RGB three colors foreach pixel, and may also include a synchronizing signal 140. A displaysignal 130 that is a second signal includes RGBW four colors dependingon the apparatus structure of the display panel.

The display panel includes RGBW four colors, and pixels each of which isa combination of RG sub pixels or BW sub pixels are arranged regularlyat pixel positions of the input signal.

The display signal 130 includes color signals in accordance with the subpixel arrangement of the display panel, and may also include thesynchronizing signal 140 in some cases. The W sub pixel does not performwavelength discrimination and therefore has transmittance that is a sumof those of sub pixels of the RGB three colors.

The transmittance when displaying white color by the RGBW four colorpanel is 1.5 times higher than the transmittance in the case of the RGBthree color panel. Practically, there is modification or variation ofnumeric values in design and production of the liquid crystal panel, butthe effect of improving transmittance by the W sub pixels may beexpected.

In order to perform display by using the RGBW panel as described above,the display signal 130 includes a combination of RGBW four colors inaccordance with the sub pixel arrangement.

The display signal conversion apparatus 120 is a unit for performing asignal conversion process of converting the input signal 110 as thefirst signal into the display signal 130 as the second signal. The inputsignal 110 is converted into the display signal 130 by the displaysignal conversion apparatus 120 so as to be transmitted to the displaypanel and output as a display screen.

The display signal conversion apparatus 120 includes a color conversionunit 150, a pixel arrangement determination unit 151, a line memory 152,a signal pattern determination unit 153, and a signal distribution unit154.

The color conversion unit 150 converts the input signal 110 from an RGBsignal into an RGBW signal.

The pixel arrangement determination unit 151 determines whether thedisplay pixel of the display panel corresponding to the target pixel tobe a conversion target includes the RG sub pixels or the BW sub pixelswhen converting the input signal 110 into the display signal 130 by thedisplay signal conversion apparatus 120.

The line memory 152 is a unit for temporarily storing the RGBW signal.

The signal pattern determination unit 153 reads the RGBW signals of thetarget pixel and of the adjacent pixels from the line memory 152 so asto determine a signal change in the pixel region as a pattern.

The signal distribution unit 154 generates a sub pixel signal based on aresult of the above-mentioned signal pattern determination and the subpixel arrangement and based on a signal processing method prepared inadvance, so as to output the sub pixel signal as the display signal 130.

The present invention may be implemented by modifying or changing theabove-mentioned structure appropriately. The pixel arrangementdetermination unit 151 may be disposed so as to operate insynchronization with the input signal 110 as illustrated in FIG. 1A, orto operate in synchronization with the display signal 130 as illustratedin FIG. 1B.

A color filter having a certain wavelength discrimination characteristicmay be disposed at the W sub pixel position of the RGBW panel.Alternatively, sub pixels of a plurality of colors that is not limitedto the four colors may be disposed. The sequence of the color conversionunit 150 and the line memory 152 may be reversed so as to perform thecolor conversion on a signal after storing the signal temporarily in thememory. Still alternatively, the line memory 152 may be formed as ascreen memory. Although not shown, a backlight may be operated withluminance modulation based on a certain condition or periodicalswitching of a light emission wavelength.

A structural example of the color conversion unit 150 is described.Here, the display signal 130 is generated from the RGB three colors ofthe input signal 110 by using the RGBW four-color sub pixels. A W signalcontains spectrum in the visible range uniformly. Therefore, the Wsignal may be generated by replacing a signal component (that is,minimum value) contained commonly in the RGB signal by W. Further, inorder to maintain the amplitude of the input signal, W is subtractedfrom the RGB three colors signal so as to be output. As a summary, thefollowing equations (1) are satisfied.

W=MIN(R,G,B)

R′=R−W

G′=G−W

B′=B−W  (1)

If RGBW (R′G′B′W) signal obtained from the equations (1) is used as thedisplay signal, the luminance of the display output is maintained.Therefore, the effect of improving luminance by adding W may not beobtained.

As other conversion equations for improving the situation, the followingequations (2) may be used.

W=MIN(R,G,B)

R′=R−W×K

G′=G−W×K

B′=B−W×K  (2)

Utilization ratio of W may be adjusted by using the coefficient K in theequations (2). If K=1, the equations (2) are the same as the equations(1).

If importance is placed on improvement of luminance, the conversionequations (3) below may be used.

W=MIN(R,G,B)

R′=R

G′=G

B′=B  (3)

Thus, luminance is improved compared with a case using original RGBsignals.

As fundamental performances of the liquid crystal display, colorreproducibility, contrast, and the like are required. However, under asituation of bright ambient environment, reflection light of externallight from the surface and light emitted from the display device aremixed, which deteriorates both color reproducibility and contrast.

Therefore, in this embodiment, there is attached more importance tocontrast rather than color reproducibility in the situation of brightambient environment, so as to improve visibility of the display screen.For this purpose, the luminance improving effect by the W sub pixels isutilized, and a unit for detecting ambient brightness and a unit forgenerating the W signal based on the ambient brightness are provided sothat a large number of W signals are generated in a brighterenvironment.

As the unit for detecting ambient brightness, a so-called illuminationsensor may be used. By using the coefficient K that is set in accordancewith ambient brightness, the RGBW signal may be calculated based on theequations (4) below.

W=MAX(R,G,B)

R′=R+(W−R)×K

G′=G+(W−G)×K

B′=B+(W−B)×K  (4)

Here, if K=0, the following equations (5) are satisfied.

W=MAX(R,G,B)

R′=R

G′=G

B′=B  (5)

Alternatively, if K=1, the following equations (6) are satisfied.

W=MAX(R,G,B)

R′=MAX(R,G,B)

G′=MAX(R,G,B)

B′=MAX(R,G,B)  (6)

By adding the W signal in this way, luminance may be improved althoughchroma saturation is lowered. The luminance improving effect becomeshighest when chroma saturation is zero, and the image becomes amonochrome (gray) image in this case. As described above, it isinevitable for the display panel viewed in a bright environment thatchroma saturation is lowered because of an influence of reflectionlight. Therefore, it is a reasonable determination, from a viewpoint ofimproving visibility, to lower chroma saturation while enhancingluminance in the signal processing as described above.

In addition, as another example of controlling the W signal, there isreduction of stand-by power consumption. For instance, if the mobileterminal is not operated for a certain period or longer, it isinstantaneously decided that the mobile terminal is in the stand-bystate with no operation, and the method of generating the display signalfor displaying on the display panel is switched.

Specifically, the display signal is generated by enhancing theutilization ratio of W in the stand-by state so as to improve thecontrast at the cost of color reproducibility. Here, the conversion fromthe RGB input signal into the RGBW display signal may be performed byusing the coefficient K that is set based on the utilization ratio of Win accordance with the equations (7) below.

W=MAX(R,G,B)

R′=R+(W−R)×K

G′=G+(W−G)×K

B′=B+(W−B)×K  (7)

According to the equations (7), it is possible to realize the displayscreen in which contrast is weighted. In addition, backlight luminanceis reduced so that power consumption may be reduced. As a combinationthereof, although backlight luminance is reduced, contrast of thedisplay signal is enhanced (utilization ratio of W is improved), so thatvisibility is secured. As a matter of course, if a certain operation isrestarted, image quality of the display panel in the operation state isrestored by preparing a procedure for getting out of the above-mentionedstand-by state. An intermediate transfer state is provided fortransferring from the operation state to the stand-by state. On thecontrary, transfer from the stand-by state to the operation state isperformed promptly, so that temporal asymmetry is provided.

Primarily, it is essential that the signal is linear for correctlyperforming addition, subtraction, multiplication, and divisionoperations. Therefore, it is reasonable from a viewpoint of linearity toperform signal processing after removing the gamma characteristic fromthe signal. However, the number of gradation steps may be decreased bythe gamma conversion, which may cause deterioration in image quality.

For instance, it is apparent that if the signal of 8 bits and 256gradation steps is processed by gamma conversion so as to be output as asignal of the same 8 bits, the number of gradation steps becomes smallerthan 256 because a signal step width becomes uneven. As a result, afalse contour may be observed in a smooth signal change region.Otherwise, balance of the RGB three colors may be lost so that anothercolor may be observed in a white color region. In addition, there is acase where grounds of linearity of a video signal (or devicecharacteristic) are not clear. Practically, the signal conversion mayoften be performed with gamma characteristic being valid. There is alsoa case where a large image quality deterioration is not observed even ifthe linearity is not satisfied. Therefore, the gamma conversion unit isnot essential in the present invention.

FIG. 3 illustrates a structural example of the pixel arrangementdetermination unit 151 illustrated in FIGS. 1A and 1B. The sub pixelarrangement of the display panel corresponding to the position of thetarget pixel is determined in advance from regularity of the sub pixelarrangement of the display panel and a position of the target pixelcalculated from the synchronizing signal 140 contained in the inputsignal 110, so that a sub pixel arrangement signal 182 is output.

As illustrated in FIG. 3, the display panel includes pixels arranged inthe pixel positions of the input signal in combination with the RG subpixels or the BW sub pixels. Further, the RG sub pixels are disposed atthe uppermost and leftmost end in the display panel, so that RG and BWare arranged alternately in the vertical direction and in the horizontaldirection.

Described below is a procedure for calculating a relationship between apixel position of the target pixel of the input signal 110 and a subpixel position of the display panel from a synchronizing signal 140 inassociation with scanning order of the input signal.

The synchronizing signal 140 includes a vertical synchronizing signalVsync, a horizontal synchronizing signal Hsync, and a pixel clock Pclk.In addition, the number of lines per screen of the input signal 110 isdenoted by Ph, and the number of pixels per line is denoted by Pv.

Similarly, regarding the display panel, the number of lines per screenis denoted by Ph, and the number of pixels per line is denoted by Pv.Write scanning is started from the upper left end.

A line number counter Vcount is counted up by the horizontalsynchronizing signal Hsync and is reset by the vertical synchronizingsignal Vsync. A pixel number counter Hcount is counted by the pixelclock Pclk and is reset by the horizontal synchronizing signal Hsync.

At the start point of the upper left end in the display panel, the linenumber counter Vcount is reset by the vertical synchronizing signalVsync while the pixel number counter Hcount is reset by the horizontalsynchronizing signal Hsync so that both counters become zero.

The circuit for realizing the above-mentioned operation may be easilyrealized by combining binary counters and the like. In this case, arelationship between the counter value and a position of the sub pixelof the display panel is as follows. If the least significant bit V0 ofthe line number counter Vcount and the least significant bit H0 of thepixel number counter Hcount are the same, the pixel is an RG sub pixel.If V0 and H0 are not the same, the pixel is a BW sub pixel.

The program is described as the expression (8) below.

IF(V0=H0) THEN RG SUB PIXEL

IF(V0≠H0) THEN BW SUB PIXEL  (8)

In this way, the sub pixel arrangement information may be generated fromthe regularity of the preset sub pixel arrangement and the synchronizingsignal contained in the input signal.

The above-mentioned description exemplifies the case where thesynchronizing signal 140 contained in the input signal 110 is used.Alternatively, it is also possible to use the synchronizing signal 140contained in the display signal 130 on the output side. There is a timeshift between the input side and the output side due to an internalcircuit structure, but they may be handled in the same manner. Theregularity of the sub pixel arrangement may be input from the displaypanel by providing a unit for holding a certain negotiation when thedisplay panel is connected.

A structural example of the signal pattern determination unit 153illustrated in FIGS. 1A and 1B is described. In the conversion from theinput signal of the target pixel set in accordance with the scanningorder into the display signal, the signal pattern of the input signalsof the target pixel and adjacent pixels is determined with reference tothe sub pixel structure of the display panel for displaying the inputsignals of the target pixel and adjacent pixels read from the linememory 152 and the target pixel output from the pixel arrangementdetermination unit 151.

In the display panel constituted of the RGBW four colors, a combinationof the RG sub pixels or the BW sub pixels is arranged at the pixelposition of the input signal alternately for each line.

There is a color that may not be displayed, depending on the sub pixelarrangement of the display panel. For instance, when displaying a fineline having the minimum line width of one pixel, the line may be brokento be in pieces so that image quality deterioration may be caused. Thisimage quality deterioration of the fine line is apt to occurparticularly in a case where the fine line has a chromatic color.

In this embodiment, in order to prevent the above-mentioned imagequality deterioration, it is judged first whether or not the inputsignal has a signal pattern that is apt to cause the image qualitydeterioration.

Here, it is assumed that the input signal has 8 bits for each of the RGBthree colors (24 bits in total) per pixel and that the display signalhas 8 bits for each sub pixel. For instance, it is necessary to refer tothe target pixel and adjacent pixels that are arranged in atwo-dimensional manner for determining whether or not a fine lineexists. If a reference region has 3 by 3 pixels, there are signalpatterns in which signals of 9 pixels including the target pixel andadjacent pixels are combined. If each pixel has a signal of 24 bits ofthe RGB three colors, there are types of ninth power of 24. If eachpixel has a signal of 32 bits of the RGBW four colors, there are typesof ninth power of 32. However, it is not necessary to handle a signalpattern that is not necessary for maintaining the image quality.Therefore, pattern types are reduced while keeping a signal pattern thatis effective for maintaining the image quality.

Here, the RGBW input signal is binarized using a threshold value so asto be converted into one bit signal. As a result, the signal of the 9pixels is converted into a combination of 9 bits, and total 512 types ofpatterns are generated. Further, because only the signal patterns thatare effective for maintaining the image quality are selected from theall types of the signal patterns and are used, the pattern types to beactually handled is reduced. In other words, the signal pattern that isapt to cause the image quality deterioration is discriminated. One ofthe feature of this embodiment is that the threshold value is set sothat a background color and a foreground color are discriminated fromeach other for enhancing accuracy of the determination.

The threshold value is set by a combination of colors, and hence it iseasily understood by considering in the color space. FIG. 4 illustratesan example of how the threshold value is set in the three-dimensionalcolor space in the case of the RGB three colors. The color space of theRGBW four colors is four-dimensional and may not be illustrated, but thesame is true for the RGBW four colors. The threshold value foridentifying yellow is set to have a large value for RG colors and asmall value for BW colors. The threshold value for identifying black isset to have a small value for each of the RGBW colors. The values may bein arbitrary sizes, a shape of a line (or a plane) connecting the valuesmay also be set arbitrarily.

The input signal is binarized by two types of threshold values for thebackground color and the foreground color so that two binary signalpatterns are generated for the background color and the foregroundcolor.

If the signals have exclusive pixel arrangements, the pixel region has asignal pattern of the background color and the foreground color. On thecontrary, if the signals do not have an exclusive pixel arrangement, thesignals are excluded because another color is mixed.

Next, it is determined whether or not the pixel arrangement is aspecific judgment pattern that is prepared in advance. Theabove-mentioned RGBW panel has a low resolution in the horizontaldirection, and hence smoothness in the vertical direction of the fineline is apt to be lost. Therefore, for example, the 3 by 3 pixelsincluding a vertical line may be prepared as the judgment pattern. Then,if the pixel arrangement matches the pattern, the process proceeds tothe signal processing procedure for maintaining the image quality. Inthe above description, the background color is yellow while theforeground color is black in the described pattern. However, anarbitrary combination of the background color and the foreground colormay be adopted.

The above-mentioned procedure may be performed by using a combinationcircuit of a comparator for comparing the input signal with thethreshold value, and an AND gate for judging matching between the binarysignal that is an output of the comparator and a pattern that isprepared in advance. The signal pattern that is prepared in advance maybe incorporated as a memory circuit or a fixed constant in the circuit.

One of the feature of this embodiment is that the signal patterndetermination unit 153 is used for detecting the pixel arrangement inwhich the background color and the foreground color included in theinput signal are combined so that the color signal conversion for apurpose of maintaining the image quality is performed in the structurein which color signal display is enabled or disabled depending on thesub pixel arrangement of the display panel.

FIG. 5 is a diagram for illustrating an operation when a fine line inthe vertical direction included in a letter or a figure of black pixelsin the yellow color background is displayed. In order to display blackby the RGBW panel, it is sufficient to turn off the drive signal simplyin both the RG sub pixels and the BW sub pixels. In order to displayyellow color, the drive signal should be turned on in the RG sub pixelposition, but it cannot be displayed in the BW sub pixel position.

As a combination of the above-mentioned structures, it is described thata case where a fine line of black color is a foreground in a yellowcolor background. When the black color fine line is displayed by turningoff the BW pixel, the background pixels sandwiching the black color fineline may display yellow color because they are the RG pixels. In thiscase, the black color fine line has a line width of one pixel. When theblack color fine line is displayed by turning off the RG pixel, thebackground pixels sandwiching the black color fine line may not displayyellow color because they are the BW pixels. In this case, the blackcolor fine line is observed as a line having a line width of threepixels.

Thus, the black color fine line alternately has the line width of onepixel and the line width of three pixels. Consequently, the result ofdisplaying is different from a smooth black line of the minimum linewidth (one pixel) intended by the input signal.

One of the feature of this embodiment including a unit for generatingthe sub pixel signal so as to display a smooth black line based on adetection result and so as to be observed as if a smooth black line isdisplayed when signals are input as described above.

In this embodiment, in order to detect a color signal, a region of acolor space represented by a combination of RGB or RGBW threshold valuesis set. In order to detect yellow, a large threshold value is set for RGcolors while a small threshold value is set for BW colors in the colorspace.

In order to detect black color, a small threshold value is set for eachof the RGBW colors in the color space. If the color signal belongs tocolor space region indicated by the threshold values, the color signalis yellow color or black color. Then, two-dimensional positions of theyellow color pixel and the black color pixel are detected with respectto the target pixel and adjacent pixels. The pixel positions of thebackground color and the foreground color are exclusive to each other,which are indicated by an inverting relationship of a binary signal.

A judgment pattern for the background and a judgment pattern for theforeground that is invert of the pattern for the background are preparedfor a predetermined pixel arrangement, and it is detected whether thebackground of the yellow color pixels as well as the foreground of theblack color pixels both match the prepared judgment patterns. Ifmatching is detected for both, it is determined that the judgmentpattern is satisfied.

As to the judgment pattern, for example, in the 3 by 3 pixel region withthe target pixel being the center pixel, there are seven types ofjudgment patterns for the black color fine line constituting a letter ora figure. Further, it is arbitrary to enlarge the pixel region or to adda judgment pattern of a signal arrangement (not shown).

A logic circuit is prepared for outputting the seven types of judgmentpatterns when a binary signal indicating an arrangement of thebackground color and the foreground color of nine pixels is input. Thejudgment patterns are not limited to the seven types in theabove-mentioned example, but may be set appropriately, and the logiccircuit is adapted for detection based on the judgment patterns thusset. The logic circuit may be realized easily by combining so-called ANDcircuits. A result of the determination may indicate matching commonlyfor all the judgment patterns or indicate matching independently foreach of the judgment patterns. In either case, the signal processing isperformed based on the determination of the matching.

FIG. 6 is a diagram illustrating an operation in a case where the inputRGB three color signals represent image data in which white color andblack color are arranged alternately. In order to detect white color, alarge threshold value is set for each of the RGBW colors in the colorspace. In order to detect black color, a small threshold value is setfor each of the RGBW colors in the color space.

A white color pixel position is detected by using a threshold value ofthe RGB combination for detecting white color, and a black color pixelposition is detected by using a threshold value of the RGB combinationfor detecting black color. If a color signal belongs to the color spaceregion indicated by the threshold values, the color signal representswhite color or black color.

A relationship between the input monochrome arrangement position and thearrangement position of the RG pixel or the BW pixel of the RGBW panelleads to a case where the input white pixel corresponds only to the RGpixel of the display panel or an opposite case where the input whitepixel corresponds only to the BW pixel of the display panel. If thedisplay panel is driven as it is in these cases, yellowish color isgenerated in the former case while bluish color is generated in thelatter case, so that white color may not be displayed in both cases.

Here, assuming that density of pixels in the display panel issufficiently high, when displaying the image data in which white colorand black color are arranged alternately, its arrangement pattern has nomeaning, and it has a meaning in that the gray color as achromatic coloris displayed. Therefore, in this embodiment, based on a criterion thatis whether white color and black color are arranged alternately in thepattern, if the criterion is satisfied, R=0, G=0, B=0, and W=1 areoutput as the display signal. In this case, the RGB sub pixels do notcontribute to displaying, and no color is generated so that achromaticcolor is displayed.

In general, a region in which pictures, letters, figures, and the likeare disposed in image data is often sufficiently larger than the 3 by 3pixels. On the other hand, the signal pattern may be determinedincorrectly based on observing the region as small as 3 by 3 pixels.

Therefore, in order to avoid the incorrect determination, it is possibleto add a condition that a single reference pixel region (3 by 3 pixelsin the above-mentioned example) is set to be larger or that the samejudgment results consecutively obtained in the adjacent regions.

Whether or not the region is continuous may be simply realized byproviding a counter for counting the number continuing the same judgmentpattern on the same line. Alternatively, the judgment result of thesingle reference pixel region for one screen is temporarily stored inthe memory so that the region of the same judgment result continuous inthe vertical direction and in the horizontal direction in the screen maybe detected. Thus, incorrect determination may be reduced throughdetermination made on a region larger than the region used for thesingle determination (3 by 3 pixels in the example described above).

In the above description, the conversion from the input RGB signal tothe RGBW signal for display is split into two steps including (1) colorsignal conversion (conversion from the RGB signal to the RGBW signal)and (2) signal conversion based on the pixel arrangement of sub pixelsof the RGBW panel of the display panel (sub pixel rendering), and isperformed in the order of (1) and (2).

As another structural example, it is possible to adopt a processprocedure in which the signal of the RG sub pixels or the BW sub pixelsis generated directly from the RGB input signal based on a result of thesignal pattern determination. In other words, it is possible to adopt astructure in which the signal of the RG sub pixels or the BW sub pixelsis generated considering the sub pixel structure of the RGBW panel fromthe beginning.

This may be realized by combining the above-mentioned steps (1) and (2)so as to structure the circuit. It is needless to say that the sameresult, as one of the object of the present invention, may be obtainedby either structure.

FIG. 7 illustrates an example of the circuit structure of the pixelarrangement determination unit 151, the line memory 152, the signalpattern determination unit 153, and the signal distribution unit 154illustrated in FIGS. 1A and 1B. Here, the RGBW signal 180 which hasalready been converted by the color conversion unit 150 is input.

The RGBW signal 180 after conversion performed by the pixel unit inaccordance with the scanning order is temporarily stored in the linememory 152. Here, a three-line structure is adopted so that thereference signal 181 of the 3 by 3 pixels region is generated from thesignal read from the line memory 152. Then, the signal arrangement ofthe pixel region is determined by the signal pattern determination unit153.

Here, as described above, there are so many combinations of signals inthis state. Therefore, by utilizing a binarization circuit 191 using athreshold value, the signal is converted into a combination signal ofthe background color and the foreground color. A plurality types ofthreshold values may be set, and M binarization circuits 191 from (1) to(M) are illustrated in FIG. 7. Each binarization circuit works so as toindicate whether or not the RGBW signal of each pixel belongs to aspecific region in the color space, as a binary signal.

A determination circuit 193 detects whether the judgment pattern 192that is prepared in advance matches with the combination signal of thebackground color and the foreground color of the above-mentioned result,and a pattern determination signal 183 is output. When determined thatit is not matched, the pattern determination signal 183 indicating nomatching is output. The judgment pattern 192, which is prepared inadvance as a plurality types of patterns that cause image qualitydeterioration, like black lines in a yellow color background, a whiteand black alternating pattern, or the like, is stored in the memory forexample or is installed as a logic circuit.

It is assumed that the input RGBW signal 180 is synchronized with thesynchronizing signal indicating the scanning order. Therefore, a pixelposition in the screen may be determined by counting based on thesynchronizing signal. In addition, a color type of the sub pixel may bedetermined from the sub pixel position in the display panel. The pixelarrangement determination unit 151 is a circuit for implementing theabove-mentioned operation, which receives the synchronizing signal andoutputs the sub pixel arrangement signal 182.

Using the pattern determination signal 183 and the sub pixel arrangementsignal 182 obtained as described above, the signal distribution unit 154generates a sub pixel signal.

As described above, a specific content of the signal processing may bedescribed directly as a method of generating the sub pixel signalcorresponding to a combination of the pattern determination signal 183and the sub pixel arrangement signal 182.

Next, a manner of operation of the signal distribution unit 154 isdescribed. The signal distribution unit 154 receives the RGBW signal 180in the reference region read from the line memory 152, the sub pixelarrangement signal 182, and the pattern determination signal 183.

FIGS. 8A and 8B illustrate an arrangement of the pixels (sub pixels) onthe display panel. It is clear that an enabled or disabled state ofdisplaying a color type exists depending on the sub pixel correspondingto the position of the target pixel. This enabled or disabled state ofdisplay is switched alternately along the arrangement of pixels. Forinstance, a fine line constituting a letter or a figure may be broken inpieces in accordance with the alternating enabled and disabled state ofdisplay, which largely deteriorates visibility.

In this embodiment, a color signal in the disabled state of display issubstituted by a different pixel or a different color so that the imagequality deterioration of being broken in pieces may be prevented. Inother words, the color signal in the disabled state of display isdistributed to a different pixel or a different color, and hence it maybe called signal distribution.

In this embodiment, the above-mentioned sub pixel structure and thesignal pattern are input so that a method of the signal distribution isdetermined. A relationship between them is prepared in advance by usinga table form, a program description, a built-in circuit, or the like.

An example of the signal distribution of the one-dimensional region isdescribed with reference to the program description. Note that thecontents of description may be easily rewritten by using a language forcircuit synthesis such as VHDL or Verilog.

IF(black color fine line exists in yellow color background)  //determination of signal pattern{   IF(target pixel == RG sub pixels) //RG sub pixels display enabled     THEN{process 1}  //distribute BWsignal to adjacent pixels   IF(target pixel == BW sub pixels)  //BW subpixels display enabled     THEN{process 2}  //distribute RG signal toadjacent pixels}

The process content for maintaining image quality when the conditionsmatch may be described in the { } of “THEN{process}”. An example of theprogram description is as follows.

                       process 1:{    W01 <= (R11 + G11)/2;    B21 <=G11/4;    //yellow color background may not be displayed in adjacentpixel (BW sub pixels),    //black color fine line may be displayed intarget pixel (RG sub pixels)    //in this state, adjacent three pixelsbecome black color display and hence line width increases,    //soadjacent W and B sub pixels are tuned on instead of yellow colorbackground. }process 2:{    NOP;  //no operation    //yellow colorbackground may be displayed in adjacent pixel (RG sub pixels),   //black color fine line may be displayed in target pixel (BW subpixels),    //in this state, black color fine line in yellow colorbackground may be displayed.}

The above-mentioned feature is to describe the signal processing methodfor preventing the deterioration when it is determined that the imagequality deterioration occurs, as a direct operation. In this embodiment,the signal processing for preventing the image quality deteriorationdescribes the signal generating method by the sub pixel unit that isthinner than one pixel of the input signal, to thereby prevent thedeterioration effectively. This is largely different from the case ofthe filtering process noting frequency components in which a signalchange corresponding to one pixel period of the input signal may not behandled correctly because of constraints of the sampling theorem.

It is needless to say that any order of the description of thecombination of the conditions may be adopted. Addition and modificationof the conditions and the process contents may be performed ifnecessary. The determination of conditions may be realized by a simplecombination of AND circuits. If the process contents exemplified aboveare used commonly in the judgment results of the different conditions,it is possible to obtain the effect of reducing a scale of the circuit.

FIG. 9 illustrates an operation of the signal distribution of theabove-mentioned one-dimensional region. Here, FIG. 9 illustrates thecase where the input signal of the fine line (having a line width of onepixel) in which the background color is a yellow color and theforeground color is a black color. (In the graphs of FIG. 9, for easyobservation, a small height is also given to the signal value of zero.)In addition, as the sub pixel position on the display panelcorresponding to the target pixel, the left side of FIG. 9 is the RG subpixels while the right side thereof is the BW sub pixels. The upper rowof FIG. 9 indicates the input signal, the middle row of FIG. 9 indicatesthe display signal obtained by the simple conversion based on the subpixel arrangement, and the lower row of FIG. 9 indicates the displaysignal obtained by the signal distribution of this embodiment.

(1) The Case where the Target Pixel Corresponds to the RG Sub Pixels

If the target pixel and the RG sub pixels are simply associated witheach other, the sub pixel signals of the three middle pixels becomezero, and hence the line width changes from one to three. Therefore, inorder to maintain the image quality, an apparent line width is made tobe narrow by the following process contents as described above.

Originally, as illustrated in graphs in the middle row of FIG. 9, R andG input signals disposed at the positions of W01 and B21 are discardedbecause there are no corresponding sub pixels of the same color.However, in this embodiment, as illustrated in the graph in the lowerrow of FIG. 9, the signal is distributed to W01 and B21 that aredifferent colors, for a purpose of maintaining the fine line. Because ofa very small region, luminance is perceived visually more than adifference of color so as to contribute to maintaining the line width ofthe fine line. process 1: { W01<=(R11+G11)/2; B21<=G11/4;}

(2) The Case where the Target Pixel Corresponds to the BW Sub Pixels

If the target pixel and the BW sub pixels are simply associated witheach other, the sub pixel signal of the center one pixel becomes zero,and hence the line width of one pixel may be maintained. Therefore, evenif the patterns match, no special process should be performed from thesub pixel arrangement condition. Therefore, the process contents may bedescribed as follows. Here, the simple signal conversion illustrated inthe middle row of FIG. 9 and the signal conversion of this embodimentillustrated in the lower row of FIG. 9 generate the same sub pixelsignal. process 2:{NOP; //no operation}

This embodiment sets the generating method of the sub pixel signaldirectly as described above, and hence the signal conversion forpreventing image quality deterioration may be performed by single subpixel unit. Contents of the signal processing corresponding to factorsof the image quality deterioration should be prepared arbitrarily. Thesignal distribution in the one-dimensional region is described above,but the distribution may be performed in the two-dimensional region, andthe generating method of the sub pixel signal may be describedsimilarly.

This signal processing is not related to the sampling theorem thatbecomes constraints in the signal processing handling frequencycomponents. Note that the circuit structure for realizing theabove-mentioned description may be rewritten by using a language such asVHDL or Verilog for circuit synthesis so that a logic circuit may beobtained easily, although not illustrated.

FIG. 10 illustrates an example of a general structural of the liquidcrystal display to which the display signal conversion apparatus of thisembodiment is added as a bridge circuit. A fundamental circuit of thedisplay is a combination of a panel drive unit 161 (so-called driver IC)and a display panel 162. The RGBW panel is connected to the fundamentalcircuit, and the display signal conversion apparatus of this embodimentis connected to the input side thereof, to thereby display the RGB inputsignal on the RGBW panel. The circuit form is disposed between theexisting RGB signal and the existing driver IC, and hence it may becalled a bridge circuit. In addition, a backlight drive unit 163 and abacklight 164 are prepared.

A W utilization ratio setting unit 155 is a unit for setting utilizationratio of W in the display screen by using a signal characteristic suchas chroma saturation, luminance, and the like obtained by measuring theinput signal, and brightness of the environment measured by the externallight sensor.

The input signal characteristic and the environmental brightness areused as a selection or a parameter of the signal conversion method inthe conversion from the input RGB signal to the RGBW signal. Forinstance, if the environment is dark, the W signal is suppressed so thatthe color reproducibility is enhanced. If the environment is bright, theW signal is increased so as to realize a bright display. In addition, ifan image region with high chroma saturation is included, the signalconversion is performed so as to suppress the W signal to be low forenlarging the color range of the display.

In particular, if the W signal is increased in a display screen in whichthe yellow color region and the white color region are mixed, yellowcolor may be observed to be dark. Therefore, if such a color combinationexists, the W signal is suppressed to be low so that the visibility ofyellow color may be improved.

In this embodiment, the W utilization ratio setting unit 155 is providedso that the corresponding utilization ratio of W is determined from acombination of the input signal characteristic and the environmentalbrightness. Thus, the improvement of the image quality as describedabove may be realized. The utilization ratio of W determined in this wayis used as a coefficient or a parameter for signal conversion in thecolor conversion unit 150 for generating the RGBW signal. It is needlessto say that the W utilization ratio setting unit 155 and the colorconversion unit 150 described above may be constituted as one circuit ora plurality of circuits in accordance with convenience of design andmanufacture.

The RGBW signal obtained by the conversion is temporarily stored in theline memory 152 so as to secure an N×N pixel region about the targetpixel. Otherwise, a screen memory may be used. The signals of the targetpixel and adjacent pixels are read from the line memory 152, and thesignal pattern determination unit 153 of the region performs the patterndetermination.

If the RGBW signal has 8 bits for each color, the number of combinationsbecomes fourth power of 8, and hence the determination circuit becomes alarge scale. Therefore, the signal of each color is binarized by anappropriate threshold value so that the pattern types are reduced. Thus,the circuit structure of the pattern determination may be simplified.The sub pixel signal corresponding to the position of the target pixelis generated from the pattern judgment result and the sub pixelarrangement by using the signal distribution unit 154. This generatingmethod may be determined in advance. The sub pixel for generating thesignal may be the sub pixel corresponding to not only the target pixelbut also the adjacent pixel. The generated sub pixel signal is output asa signal for displaying and is transmitted to the RGBW panel via thepanel driving circuit for the display operation.

Here, the luminance modulation of the backlight may be performed at thesame time. For this purpose, backlight luminance necessary for displayis calculated by using a result of measuring signal distribution of theinput RGB signal or the RGBW signal for displaying. Then, the backlightdrive unit 163 generates the drive signal based on pulse widthmodulation (PWM), for example, so as to drive the backlight 164.Although not illustrated, a unit for modifying the display signal so asto be combined with the set backlight luminance for display is prepared.

As another embodiment, a structural example of a television receiverutilizing the RGBW panel is described. As fundamental requirements as adisplay, there are reduction of power consumption, increase of contrast,improvement of color reproducibility, and the like. In addition to them,as a specification unique to a television, many pixels are arranged in alarge screen. The screen resolution of a high-definition television haspixels of 1,920 by 1,080 at most.

On the other hand, there are various signal resolutions of the videosignal, and the signal resolution is different between an analogtelevision and a high-definition television. In addition, the videosignal resolution is different between terrestrial digital broadcastingand BS digital broadcasting.

In this way, it is well known that, as to a television, the signalresolution of the video signal is different from the screen resolutionof the display panel, and it is a well-known technology to useresolution conversion for solving the above-mentioned problem.

In this embodiment, as one type of the above-mentioned resolutionconversion, the display panel in which the RGBW sub pixels are arrangedis used so as to insert the signal conversion from the input RGB signalto the display signal for the RGBW sub pixels. FIGS. 11A and 11Billustrate a structural example of a television receiver. As illustratedin FIGS. 11A and 11B, the television receiver includes a TV tuner 131, aletter/figure generation unit 132, and a screen synthesis unit 133.

It is rare that a usual television video signal contains a signal of aminimum line width corresponding to only one pixel of the displaydevice. This is because of a resolution characteristic of a camera or afactor that a letter or a figure to be superimposed and synthesized isrelatively large.

On the other hand, the video signal of a letter or a figure such as aprogram table or a menu created in the television set may have anarbitrary line width by setting in the generation circuit inside.Therefore, if the internally generated video signal is created so as tohave substantially the same frequency characteristics as those of thebroadcasting video signal, visual feelings about resolutions of themdisplayed on the display may be matched with each other.

In this embodiment, the minimum line width of a letter or a figure ofthe internally generated chromatic color is set to be larger than onepixel. Specifically, the minimum line width of a letter or a figure ofthe internally generated chromatic color is set so that the sub pixelsof all color types of the display panel are included on the line widthof the fine line when the fine line is displayed.

In the example of the above-mentioned pixel arrangement of the RGBW subpixels, the minimum line width corresponds to one pixel in the verticaldirection and two pixels in the horizontal direction. When the minimumline width is set in this way, all the RGBW sub pixels are included onthe line width. Therefore, a letter or a figure of an arbitrary colormay be displayed.

On the other hand, a fine line of black color may be displayed even bythe single sub pixel, but in order to keep a balance of the line widthwith the fine line of a chromatic color, it may be the same value as theminimum line width of the chromatic color.

If the minimum line width is set in this way, color reproducibility ofthe fine line may be improved.

In addition, if a signal correction is performed by the above-mentionedsignal pattern determination, it is possible to realize the effect ofimproving the image quality in which smoothness of the fine line isenhanced. It is needless to say that the above description exemplifiesthe television as an application, but the present invention may beapplied to other display devices. For instance, if a monitor screen of amobile terminal, a digital camera, or the like includes the RGBW panelof the above-mentioned sub pixel arrangement, it is possible to realizea display having improved visibility enhanced in which a letter or afigure of an internally generated operating screen is made to have aminimum line width of at least one pixel in the vertical direction andtwo pixels in the horizontal direction.

As one technology for realizing low power consumption, there is a knowntechnology of modulating the backlight luminance based on brightness ofambient environment and contents of the video signal. Supposing that thescreen size is large in an application for television, it is relativelyeasy to divide the backlight into a plurality of regions in the screen.Then, by luminance modulation for each region, it is possible to obtaineffects of improving contrast, reduction of power consumption, and thelike.

The display output that passes through the W sub pixel of the liquidcrystal panel depends on the light emission wavelength of the backlight.Strictly speaking, wavelength characteristics of the liquid crystalelement are added, but it is omitted for easy description. On the otherhand, the RGB sub pixels become the display output in which the lightemission wavelength of the backlight is combined with the transparentwavelength of the sub pixel. Here, in order to realize reduction ofpower consumption, improvement of contrast, improvement of colorreproducibility, and the like, which are required to the display, it ispossible to devise the backlight characteristic in addition toimprovement of the characteristics of the liquid crystal panel.

In order to reduce the power consumption, it is effective to calculateminimum light emission quantity necessary for display the input signalso as to modulate the backlight luminance for driving. The backlightluminance modulation is effective for improving contrast because themaximum luminance is maintained while the minimum luminance in the darkscreen is suppressed to be low. In addition, if the light emissionwavelength distribution of the backlight is appropriately adjusted, theeffect of enlarging color reproducing range is obtained. Theabove-mentioned structure may be used in combination with the displaypanel having the RGBW sub pixel structure.

FIG. 12 illustrates an example of an apparatus structure for performingthe backlight luminance modulation. In this embodiment, the signal fordriving the display panel is generated by the signal distribution unit154. Therefore, the signal is input to a backlight luminance settingunit 171 so as to calculate the backlight luminance necessary for thescreen display.

Then, in order to maintain the luminance of the display screen whileusing the calculated backlight luminance, the display signal iscorrected by using the display signal correction unit 172. Thus, theimage quality is maintained by the combination of the backlight and thedisplay panel while the power for driving the backlight is reduced. Inaddition, the backlight luminance is reduced for a dark screen, andhence the effect of improving contrast is obtained.

Note that, although not illustrated, it is possible to prepare anillumination sensor so as to adjust the backlight luminance based onenvironmental brightness. Further, although not illustrated, informationof color type and color arrangement of the display panel is transmittedto the backlight luminance setting unit 171 and the display signalcorrection unit 172 in advance. The color type and the color arrangementof the display panel are not limited and are arbitrary.

The light source unit of the backlight is not limited and may be, forexample, a white color LED, a combination of RGB three color LEDs, or acombination of light sources of four or more colors. In addition, thebacklight luminance may be controlled integrally in the entire screen orin each of appropriately divided regions. The signal processing unit formaintaining color reproducibility is prepared in the backlight luminancesetting unit 171 and the display signal correction unit 172, and henceimage quality is maintained while power consumption for driving thebacklight may be reduced.

The present invention does not restrict the color type of the sub pixelof the display panel. In the above-mentioned embodiment, the RGBW fourcolor display panel with the additional W sub pixels is described, butit is possible to add sub pixels with a color filter of cyan, yellow, orthe like instead of W (white). In addition, the number of colors is notlimited to four colors and may be five colors, six colors, or the like.In either case, the signal processing of generating the sub pixel signalaccording to this embodiment is performed based on the rule of the subpixel arrangement, and hence improvement of transmittance or colorreproducibility may be achieved.

FIG. 13A illustrates a display panel in which RGBW color filters arearranged regularly as color types with the same existing RGB sub pixelarrangement of the display panel. In this RGBW display panel, color typecombinations of the sub pixels corresponding to the pixel positions ofthe input RGB signal are four types of RGB, GBW, BWR, and WRG.

Then, combinations of four type sub pixels are repeated at a period ofthe four pixels. In the horizontal direction, the same color appearsevery four sub pixels. In the vertical direction, the same color appearsevery other line in this example, and periods of individual colors inthe two-dimensional position are uniform. However, the color type andthe color arrangement of the sub pixels are not limited to thisstructure.

In this example, because each of RGBW colors is included three times inthe period of four pixels as RGB transmittance is 1 and W transmittanceis 3, the total is (1(R)+1(G)+1(B)+3(W))×3=18.

On the other hand, in the RGB display panel, because each of the RGBcolors is included four times in the period of four pixels, the total is(1(R)+1(G)+1(B))×4=12. Comparing the RGBW display panel and the RGBdisplay panel, it is understood that transmittance may be improved by1.5 times.

The W signal may use MIN(R, G, B), MAX(R, G, B), or a value obtained bymultiplying a coefficient K to one of them, or the like, as describedabove. The coefficient K may be set in a variable manner based on thechroma saturation of the signal, the environmental brightness of thesignal, or the like.

The input RGB signal and the RGBW signal for displaying have differentpixel arrangements. Therefore, the signal conversion unit of thisembodiment is used for generating the sub pixel signal for displaying.

First, using the pixel arrangement determination unit 151, the type ofthe four types of the sub pixel structure is determined. It is assumedthat the synchronizing signal 140 includes the vertical synchronizingsignal Vsync, the horizontal synchronizing signal Hsync, and the pixelclock Pclk.

The line number counter Vcount counts up the horizontal synchronizingsignal Hsync, and the pixel number counter Hcount counts up the pixelclock Pclk. Each of them has a value 0 at the uppermost and leftmostposition.

In this case, the positional relationship between the counter value andthe sub pixel of the display panel is expressed as follows, where theleast significant bit of the line number counter Vcount is denoted byV0, and the two lower bits of the pixel number counter Hcount aredenoted by H1 and H0.

   IF(V0=0){    IF(H1=0)&(H0=0)) THEN RGB SUB PIXEL    IF(H1=0)&(H0=1))THEN WRG SUB PIXEL    IF(H1=1)&(H0=0)) THEN BWR SUB PIXEL   IF(H1=1)&(H0=1)) THEN GBW SUB PIXEL} ELSE IF(V0=1){   IF(H1=0)&(H0=0)) THEN BWR SUB PIXEL    IF(H1=0)&(H0=1)) THEN GBW SUBPIXEL    IF(H1=1)&(H0=0)) THEN RGB SUB PIXEL    IF(H1=1)&(H0=1)) THENWRG SUB PIXEL}

Then, the RGBW signals of the target pixel and the adjacent pixelsstored in the line memory 152 temporarily is read out, whether the RGBWsignals are matched with the judgment pattern that is prepared inadvance by using the signal pattern determination unit 153 is detected,and based on a result of the judgment pattern and the sub pixelstructure, the signal distribution unit 154 may generate the sub pixelsignal. Here, the generating method of the sub pixel signal in thesignal distribution unit 154 is determined in advance so as to preventthe image quality deterioration.

A merit of this sub pixel arrangement is in that the existing technologyof a display panel of RGB sub pixels arrangement may be utilized.

By only changing the color type of the color filter of the displaypanel, and adding the display signal conversion apparatus of thisembodiment, the signal conversion for generating the sub pixel signal ofthis embodiment is performed. In this embodiment, electric and physicalcharacteristics of the input signal 110 and the display signal 130 aremaintained. Therefore, the existing RGB display panel and theillustrated RGBW display panel may be connected to each other in acompatible manner.

The signal conversion of this embodiment based on the color type and thecolor arrangement of the sub pixel of the display panel is provided.Therefore, the same display screen may be obtained by both panels. It ispossible to provide a unit for negotiating the color type and the colorarrangement of the sub pixel of the display panel when the display panelis connected. Thus, the RGBW panel that is easily manufactured and thedisplay signal conversion apparatus of this embodiment having the simplecircuit structure are combined, so as to improve the transmittance. Inaddition, by combining with the luminance modulation unit of thebacklight, reduction of power consumption and improvement of contrastmay be realized.

FIG. 13B illustrates an example where a yellow color filter is disposedinstead of the W (white color) filter. With an apparatus structuresimilar to that described above, the signal conversion for generatingthe sub pixel signal may be performed. This display panel may becombined with a multicolor backlight with modulatable luminance so as toperform display.

As illustrated in FIGS. 14A and 14B, the order of the color conversionmay be changed. A result of the signal pattern determination unit 153may be input to the color conversion unit 150 and may be used foradjusting or switching a method of processing the color conversion.Thus, the method for the color conversion may be switched between aregion having a small signal change like a photograph and a regionhaving a large signal change like a letter or a figure, for example. Forinstance, as the generating method of the W signal, the coefficientsetting method in the conversion based on amplitude of the signal, asdescribed above may be used.

W=MAX(R,G,B)×K

W=MIN(R,G,B)×K

Here, the coefficient K is set based on a result of the signal patterndetermination unit 153. As to the coefficient K, a two-dimensionalsignal change is considered so that roughness due to turning on of the Wsub pixel does not appear in the photograph region, and a signal changeof the sub pixel unit is stored in the letter or figure region. Thesignal pattern determination unit 153 utilizes the difference betweenthe photograph region and the letter or figure region for detecting afine line or an edge, and hence the two-dimensional signal change in theimage region may be detected.

As described above, the present invention made by the inventors of thepresent invention is specifically explained based on the embodiments,but the present invention is not limited to the embodiments. The presentinvention may be modified variously within the scope of the presentinvention without deviating from the spirit thereof.

What is claimed is:
 1. A display device comprising: a display panelincluding red, blue, green and white (RBGW) sub pixels; a signalconversion apparatus for converting an input signal having a first colorformat and color arrangement into a display signal having a second colorformat and color arrangement; a color conversion unit configured toconvert the first color format into the second color format; a sub pixelposition determining signal which determines a color arrangement of asub pixel position of the display panel; and a sub pixel signalgeneration unit configured to generate a sub pixel signal having thesecond color format and color arrangement based on the sub pixelposition determining signal.
 2. The display device according to claim 1,wherein the input signal comprises a signal of disposing RGB sub pixelsat individual pixels, and wherein the display signal comprises a signalof disposing one of RG sub pixels and BW sub pixels at individualpixels.
 3. The display device according to claim 1, wherein the colorformat of the display signal comprises RGBW, and wherein the displaysignal is associated with the sub pixel position of the display panel.4. The display device according to claim 1, wherein the sub pixelposition determining signal is set by a signal of an adjacent pixel. 5.The display device according to claim 2, wherein the color conversionunit converts RGB of the input signal into RGBW of the display signal,and wherein the color conversion unit controls an amplitude of a Wsignal based on the sub pixel position determining signal.
 6. Thedisplay device according to claim 2, wherein the color conversion unitconverts RGB of the input signal into RGBW of the display signal, andwherein the color conversion unit controls an amplitude of a W signalbased on environmental brightness.
 7. A display device comprising: adisplay panel including red, blue, green and white (RBGW) sub pixels; asignal conversion apparatus for converting an input signal having afirst color format and color arrangement into a display signal having asecond color format and color arrangement; a color conversion unitconfigured to convert an input signal into an display signal; the colorformat of the display signal comprises RGBW; a sub pixel positiondetermining signal which determines a color arrangement of a sub pixelposition of the display panel; and a sub pixel signal generation unitconfigured to generate a sub pixel signal having the second color formatand color arrangement based on the sub pixel position determiningsignal.
 8. The display device according to claim 7, wherein the subpixel signal generation unit generates the sub pixel signal of a targetpixel into one of a different pixel and a different color based on thesub pixel position determining signal.
 9. The display device accordingto claim 7, wherein the sub pixel position determining signal includesthe signal pattern determination signal of a target pixel and a subpixel arrangement signal.